Porous membranes are of interest for various applications, such as fuel cells, gas separation and gas sensing. Porous membranes are often used as active and/or as supporting layers in fuel cells, e.g., as considered in U.S. Pat. Nos. 5,998,058, 6,720,105, 6,743,543, and US 2004/0013924. Examples of application of porous membranes for gas separation include U.S. Pat. Nos. 4,857,080 and 5,498,278. Further examples of conventional porous membranes include U.S. Pat. Nos. 5,734,092, 6,027,796 and 6,562,446.
Although both metallic and non-metallic porous membranes are known, metallic membranes are preferred for many applications. For example, a metallic support membrane for a fuel cell provides an attractive combination of electrical conductivity and mechanical durability (e.g., resistance to cracking and breaking). Exemplary discussions of porous metallic membranes include U.S. Pat. Nos. 6,368,751, 6,649,559, 6,797,422 and US 2002/0028345. Various methods are known for fabricating porous metallic membranes. For example, US 2002/0028345 considers cold working a pore-free composite of two compositions, where one of the compositions is removed after the cold working to provide a porous metallic structure. In U.S. Pat. No. 6,649,559, removal of one phase of a two phase composite to provide a porous metallic structure is considered. In U.S. Pat. Nos. 6,338,751 and 6,797,422, commercial availability of porous metal foams is indicated.
Porous metallic membranes have also been fabricated using a two step replication process. This replication process begins with a non-metallic porous template. A negative of this template is formed by filling the pores of the template with a suitable negative material. After filling the pores, the original template is removed, exposing the negative. The desired metallic composition is then deposited on the negative, and the parts of the negative that initially filled the pores of the template form corresponding pores in the metallic structure. Finally, the negative is removed to provide the porous metallic membrane.
Such two step replication has been considered by various authors. References include: Lei et al., “Preparation of highly ordered nanoporous Co membranes assembled by small quantum-sized Co particles”, J. Vac. Sci. Tech. vB19(4), pp 1109-1114, 2001; Masuda et al., “Ordered Metal Nanohole Arrays Made by a Two-Step Replication of Honeycomb Structures of Anodic Alumina”, Science v268, pp 1466-1468, 1995; Masuda et al., “Fabrication of Pt microporous electrodes from anodic porous alumina and immobilization of GOD into their micropores”, J. Electroanalytical Chem., v368, pp 333-336, 1994; Masuda et al., “Fabrication of Porous TiO2 Films using two-step replication of microstructure of anodic alumina”, Jpn. J. Appl. Phys. v31(12B) pt 2, pp L1775-L1777, 1992; Masuda et al., “Preparation of Porous Material by Replacing Microstructure of Anodic Alumina Film with Metal”, Chem. Lett., pp 621-222, 1990; Jiang et al., “Electrodeposited Metal Sulfide Semiconductor Films with Ordered Nanohole Array Structures”, Langmuir v17, pp 3635-3638, 2001; and Jiang et al., “Ordered Porous Films of Organically-Modified Silica Prepared by a Two-Step Replicating Process”, Colloids and Surfaces A v179, pp 237-241, 2001.
Two step replication is most useful in cases where it is easier to obtain the desired pore geometry in a non-metallic template material rather than directly in the metallic composition of interest. However, difficulties have been noted in the art relating to two step replication, e.g., in structures having high aspect ratio pores.
It is generally desirable to have a large degree of control over the pore geometry. For example, it is often desirable to combine high gas transfer rate with small pore size in a mechanically sturdy (i.e., relatively thick) membrane. For a single-stage membranes having the same pore size throughout the membrane, these goals are incompatible, thus requiring a designer to make a compromise. However, this problem can be addressed by making use of a two-stage membrane, having a thin small-pore region and a thick large pore-region. Such a two-stage membrane can simultaneously provide mechanical strength in combination with small pore size and high flow rate. The general idea is similar to the known idea of providing a suitable mechanical support structure for a permeable membrane (e.g., as considered in U.S. Pat. No. 3,505,180).
However, for micro-porous membranes, macroscopic fabrication approaches (e.g., as considered in U.S. Pat. No. 3,505,180) are generally inapplicable. Examples of known two-stage microporous membranes include U.S. Pat. Nos. 5,114,803, 5,262,021, 5,308,712, and 5,620,807. A noteworthy feature of these examples is that they all relate to non-metallic membranes. Also, as expected for microfabrication technology, the fabrication techniques employed tend to be material-specific. Non-metallic two-stage membranes are also commercially available. For example, some of the Anodisc® membranes provided by Whattman are two-stage alumina membranes. However, we are not aware of any example of two-stage microporous metallic membranes in the art.
Accordingly, it would be an advance in the art to provide metallic two-stage porous membranes, and to provide methods for making such membranes.